In power plants of the above-mentioned kind, the wall of the combustion chamber is subjected to large forces. Between the space in the pressure vessel around the combustion chamber and the space inside the combustion chamber, a pressure difference arises in operation, which is due--on the one hand--to the resistance in the nozzles for the supply of air for fluidization of a bed material in the bottom part of the combustion chamber, and--on the other hand--to the resistance within the fluidized bed. This pressure difference may amount to the order of magnitude of 0.1 MPa (1.0 bar). The combustion chamber wall(s) may have an area of 10.times.20 m, so the forces which act on the wall(s) of the combustion chamber are very large, and pose constructional problems which are difficult to solve. In addition, the bed is very heavy and its operating temperature is high, (750.degree.-950.degree. C.). The forces arising because of the pressure difference between the inner and outer sides of the combustion chamber can be taken up by a framework. By thermally insuling of the combustion chamber and cooling the framework with combustion air the temperature of the framework is allowed to be kept low enough--below about 300.degree. C. that its strength is not impaired.